PAYE & National Insurance – Has the employment income been charged correctly?

I’m a 21 year old, part-time student in the UK who was employed for 7 months last year and paid on a monthly basis. Total gross income when I left was £3,523.96. PAYE tax of £56.80 and National Insurance of £75.90 was deducted from the total gross income.

It’s to my understanding that for 2009/10, national insurance is only paid when cash earnings exceed a threshold of £5,715. My PAYE code is 647L, so that means I’m entitled to a tax free pay of £6,475???

When I received my P45 when I left employment the national insurance number on the form wasn’t even mine. I have a feeling I’ve been over-taxed?? Just wanted to make sure i was right before I took this matter to the tax office